Recovery of substantially pure soda ash from a trona ore deposit typically involves either hard rock, underground mining or solution mining, with either recovery technique involving substantial aboveground processing to remove impurities, both inorganic and organic compounds, to yield a final crystalline product that is substantially white in color and with a purity in excess of 99% by weight sodium carbonate.
Trona is a naturally occurring ore that is a double salt Na2CO3.NaHCO3.2H2O, also referred to as “sodium sequicarbonate.” Inorganic impurities contained in shales associated with the trona deposit, such as sodium, calcium, and magnesium compounds, and silicates, etc., are often present in the ore in quantities of from about 9.0% to about 19.0% by weight and also contain about 1.5% to 2.0% free water. Also, carbonaceous impurities from the shale and in the trona ore itself totaling up to about 0.20% by weight are also typically present.
Both the organic and inorganic impurities are, desirably, either eliminated or substantially diminished during the aboveground processing and purification processing.
Large deposits of trona are located in southwestern Wyoming. Other deposits are located in Turkey, Africa, China and Mexico. About 90% of soda ash produced in the U.S. comes from the trona deposits in Wyoming. The trona deposits are often found in conjunction with shale oil deposits above or below the trona bed. This generally explains the carbonaceous impurities interlaced with the trona ore.
Typical aboveground processing is to crush the raw trona ore, which, after screening, is then calcined, at temperatures generally greater than 150° C. (302° F.), to convert a substantial quantity of sodium bicarbonate to sodium carbonate. The calcined product is then dissolved in a weak liquor. As used herein, the phrase “weak liquor” refers to an aqueous solution containing between 2.0% to 10.0% Na2CO3 by weight. The dissolved calcined product may then be clarified and filtered to remove insoluble shale solids and insoluble inorganics such as clays, dolomites, shortites and silicates. Carbonaceous impurities, some of which may be volatilized during the calcining step, remain, with a significant portion of these soluble in the aqueous medium. Treatment with activated carbon to absorb organic impurities is typically done so that the organic impurities may be removed by further filtration. These organic impurities, besides adversely affecting the color of the final soda ash product, cause moderate to severe foaming during subsequent evaporative crystallization processes that form mono-hydrate sodium carbonate crystals from the dissolved calcined product. Optional pretreatment of the dissolved calcined product with activated carbon and anti-foaming additives necessary to reduce foaming during the crystallization processes are expensive and sometimes not effective. Severe foaming may also cause expensive mechanical damage to the crystallizer, such as, for example, demister pads and mechanical vapor recompression equipment of the crystallizer. Severe foaming may also adversely affect triple-effect evaporative crystallizers causing sodium carbonate liquor carry over and contamination into the condensate system and cooling tower that leads to a loss of vacuum and boiling in the crystallizer bodies, which decreases production. Measures to control such foaming events, such as increasing the crystallizer liquor purge rate, also have a significant cost impact.
Regardless of the trona ore recovery technique (underground mining or solution mining) troublesome carbonaceous impurities are present that complicate and increase the expense of producing essentially pure, substantially white sodium carbonate.